1. Field of the Invention
This invention relates to the surface passivation of III-V type semiconductors and more particularly to the passivation of indium gallium arsenide
2. Description of the Prior Art
Gallium arsenide and other III-V type semiconductors are attractive candidate materials for the fabrication of high performance semiconductor components. Although such materials offer the promise of high operational speed, they generally suffer from having surfaces with poor electronic qualities. One parameter which characterizes surface quality is the surface recombination velocity (S.sub.o). Silicon surfaces (S.sub.o .about.100 cm/s) can be passivated by the development of a thermal oxide layer. However, attempts to surface passivate the III-V type semiconductors (S.sub.o .about.10.sup.6 cm/s for GaAs) have not been entirely successful. The native oxide layers which can be formed on III-V compounds exhibit significant charge trapping under bias stress and, therefore, these native oxides are relatively ineffective as the surface passivation agent. In the case of gallium arsenide, growth of a native oxide layer leads to the formation of extrinsic defects yielding a high surface state density. In addition, most native III-V oxides are susceptible to environmental attack, for example, by moisture and oxygen.
Known surface passivation methods for III-V compound semiconductors which are made typically of GaAs may be divided roughly into three types.
The first method utilizes deposited films such as SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and P.sub.2 O.sub.5 which are known from their use as passivation films for the surface of silicon semiconductors. Such an approach has the drawback that the deposition temperature is relatively high. SiO.sub.2 film is most frequently used in view of the extensive practical knowledge concerning the deposition of such films in planar silicon semiconductor devices. However, SiO.sub.2 films tend to absorb Ga from the surface of a substrate made of GaAs or GaP, and as a result such layers have the tendency to damage the stoichiometry of the surface of the substrate, leading to a high density of surface states and a large S.sub.o.
The second method is to form a native oxide film corresponding to a thermal oxidation film of silicon, in place of the deposited film suggested above. For example, the anodic oxidation method has the advantage that an insulating thin film can be formed at a markedly low temperature as compared with the deposition method and also with the thermal oxidation method, irrespective of the instances wherein a solution is used or a gas plasma is used. Conversely, however, this anodic oxidation method has the disadvantage that it is thermally unstable, and therefore, it has the drawback that the quality of the film will change substantially at a temperature below the temperature range adopted for thermal diffusion of impurities and post-ion implantation annealing. Furthermore, the interface between an anodic oxide film and a substrate made of GaAs or GaP tends to contain a number of defects, so that when this film is utilized as an insulating film of an IGFET (insulated - gate field effect transistor), there still cannot be obtained as yet a large value of surface mobility comparable with that within the bulk, and thus at the current technical stage, it is not possible for the anodic oxide film to fully display those advantages and features on applying it to the surface of GaAs and GaP substrates which are represented by high mobility as compared with a silicon substrate. In III-V semiconductors which essentially are binary compounds, a direct thermal oxidation of their surfaces has not yet produced any satisfactory results with respect to the quality of the film produced or to the state of interface. Such native oxide film has the further drawback that it is dissolved in acids such as HF, HC1, and H.sub.2 SO.sub.4. Therefore, native oxide films inconveniently cannot be used in such manufacturing process as would comprise a number of steps.
The third approach is to perform chemical oxidation by the use of, for example, hot hydrogen peroxide solution. This method is entailed by limitation in the thickness of the oxide film which is formed, and accordingly the extent of application of this method is limited also.
The use of sulfides in connection with the passivation of semiconductor substrates is disclosed in two U.S. patents. U.S. Pat. No. 4,320,178 describes the use of an A.sup.III B.sup.V sulfide for passivating an A.sup.III B.sup.V semiconductor substrate, such as gallium arsenide. The sulfide is formed by a process of heating the substrate with sulfur or hydrogen sulfide.
U.S. Pat. No. 4,632,886 describes the use of an electrolyte solution of sulfide ions to provide a chemically passivation layer on mercury cadmium telluride semiconductor substrates. The description is limited to a discussion of that specific compound semiconductor, and the passivation layer is described as being mercury sulfide, cadmium sulfide, and tellurium sulfide.
U.S. patent application Ser. Nos. 021,667 and 021,668 assigned to Bell Communications Research, Inc., the assignee of the present application, describe the use of a sulfide film consisting of Na.sub.2 S. 9H.sub.2 o as a passivation layer on III-V semiconductor substrates. However, prior to the present invention there has not been a simple, easily implemented surface passivation technique for indium gallium arsenide specifically which has been shown to achieve a sufficiently low surface recombination velocity for practical device applications.